2-Acetylaminofluorene and benzo(a) pyrene were activated to frameshift mutagens in the Ames-Salmonella test system (TA 1538) by rat liver tissue microsomes but not by dog testicular tissue microsomes. Both tissues possess cytochrome P-450, although testicular tissue has a much smaller amount than does liver. Testicular microsomes did not metabolize benzo(a) pyrene to any great extent and did not activate this compound to a frameshift mutagen. Mitotane, o,pDD was not activated by dog adrenal microsomes to a frameshift mutagen.